A cellulose acetate film is used in various photographic or optical elements because it is tough and has enough flame retardant properties. The cellulose acetate film is a representative photographic support. Further, the film is also used in a liquid crystal display. The cellulose acetate film has a high optical isotropy (giving relatively low retardation) compared with other polymer films. Accordingly, an optical isotropic film such as a protective film of a polarizing plate usually comprises the cellulose acetate film.
On the other hand, an optical compensatory sheet (phase retarder) in a liquid crystal display must have optical anisotropy (must give high retardation). Accordingly, a film of synthetic polymer (such as polycarbonate or polysulfone) giving high retardation is normally used as the compensatory sheet. In practice, the film of synthetic polymer is stretched to prepare a stretched birefringent film for the optical compensatory sheet.
As described above, generally in the technical field of optical elements, the synthetic polymer film is used as the element that must have optical anisotropy (must give high retardation), while the cellulose acetate film is used as the element that must have optical isotropy (must give low retardation).
In contrast, European Patent No. 0911656A2 discloses a cellulose acetate film giving such high retardation that it can be used as an optical anisotropic element. A liquid crystal display can give an image of high quality where the cellulose acetate film is provided between a polarizing plate and a liquid crystal cell.
In place of the stretched birefringent film, an optical compensatory sheet comprising a transparent support and a thereon provided optically anisotropic layer comprising discotic molecules has been proposed. The optically anisotropic layer is formed through the steps of aligning the discotic molecules and then fixing the alignment. The discotic molecules give large birefringence and have various alignment forms, and accordingly an optical compensatory sheet obtained from the discotic molecules has a specific optical characteristic that cannot be obtained from the conventional stretched birefringent film. The optical compensatory sheet comprising discotic molecules is described in Japanese Patent Provisional Publication No. 6(1994)-214116, U.S. Pat. Nos. 5,583,679, 5,646,703 and German Patent Publication No. 3,911,620A1.
The optical compensatory sheet is also used in a λ/4 plate, which is widely used in an anti-reflection film or a liquid crystal display. However, in spite of the name of λ/4, most λ/4 plates give λ/4 at particular wavelengths.
Japanese Patent Provisional Publication Nos. 5(1993)-27118 and 5(1993)-27119 disclose a λ/4 plate in which a birefringent film giving high retardation and another birefringent film giving low retardation are laminated so that their optical axes may be perpendicularly crossed. If the retardation difference of those films is kept λ/4 in the whole visible wavelength region, the λ/4 plate theoretically gives λ/4 in the whole visible wavelength region.
Japanese Patent Provisional Publication No. 10(1998)-68816 discloses a λ/4 plate giving λ/4 in a wide wavelength region. The disclosed λ/4 plate comprises laminated two films made of the same polymer, and one of the films gives λ/4 and the other gives λ/2 at the same wavelength. Japanese Patent Provisional Publication No. 10(1998)-90521 also describes another wide-ranging λ/4 plate comprising laminated two polymer films. As the polymer film used in the above disclosed λ/4 plates, a stretched film of synthetic polymer such as polycarbonate is used.
A liquid crystal display (hereinafter often referred to as LCD) is widely employed instead of CRT (cathode ray tube) because of its small thickness, lightweight, and low power consumption. The liquid crystal display generally comprises a pair of polarizing plates and a liquid crystal cell provided between them. The liquid crystal cell comprises a pair of substrates, rod-like liquid crystal molecules and an electrode layer. The rod-like liquid crystal molecules are provided between the substrates, and the electrode layer has a function of applying a voltage to the rod-like liquid crystal molecules. On each substrate, an orientation layer is provided to align the rod-like liquid crystal molecules. Each polarizing plate comprises a pair of transparent protective films and a polarizing membrane provided between them.
In the liquid crystal display, an optical compensatory sheet (phase retarder) is often provided between the liquid crystal cell and the polarizing plate, to prevent the displayed image from undesirable coloring. The layered composition of the polarizing plate (or polarizing membrane) and the optical compensatory sheet serves as an elliptically polarizing plate. The optical compensatory sheet also often enlarges a viewing angle of the liquid crystal cell. As the optical compensatory sheet, a stretched birefringent polymer film has been conventionally used.
In a TFT (thin film transistor) liquid crystal display of TN (twisted nematic) mode (which is widely and mainly used), the optical compensatory sheet is provided between the liquid crystal cell and the polarizing plate (in the manner described in Japanese Patent Provisional Publication No. 8(1996)-50206) to ensure an image of high quality. However, that liquid crystal display is often made relatively thick.
Japanese Patent Provisional Publication No. 1(1989)-68940 discloses another liquid crystal display. In the liquid crystal display, the optical compensatory sheet is provided on one surface of the polarizing membrane, and on the other surface an elliptically polarizing plate having a protective film is provided. That liquid crystal display can be made relatively thin, and can give an image with high contrast when seen frontally. However, the optical compensatory sheet in the display is often deformed (for example, by heat) to cause phase retardation. If so, when the display gives a black image, the transmittance at the peripheral part of the screen so increases that the displayed image is framed with leaked light. Thus, the liquid crystal display has insufficient durability.
In order to solve the above problem, Japanese Patent Provisional Publication No. 7(1995)-191217 and European Patent No. 0911656A2 disclose another liquid crystal display. In the disclosed display, an optically anisotropic layer comprising a discotic compound is provided on a transparent support to form an optical compensatory sheet serving as the protective film of the polarizing plate. That liquid crystal display can be made relatively thin, and has improved durability.
U.S. Pat. Nos. 4,583,825 and 5,410,422 disclose a liquid crystal display comprising a liquid crystal cell of bend alignment mode in which rod-like liquid crystal molecules in upper part and ones in lower part are aligned essentially in reverse (symmetrically). Since rod-like liquid crystal molecules in upper part and ones in lower part are symmetrically aligned, the liquid crystal cell of bend alignment mode has self-optical compensatory function. Therefore, this mode is referred to as OCB (optically compensatory bend) mode. The liquid crystal display of bend alignment mode has an advantage of responding rapidly.
The liquid crystal display of bend alignment mode responds rapidly, and gives a clear image within a wide viewing angle, as compared with a generally used display (e.g., a display of TN mode or STN mode). However, in order to take the place of CRT, the display of bend alignment mode must be further improved.
To improve the display of bend alignment mode, the optical compensatory sheet (which is generally used to improve a conventional display) may be used. However, a conventional compensatory sheet of stretched birefringent film cannot fully compensate the display of bend alignment mode.
As described above, an optical compensatory sheet comprising a transparent support and a thereon provided optically anisotropic layer comprising discotic molecules has been proposed in place of the stretched birefringent film. In fact, Japanese Patent Provisional Publication No. 9(1997)-197397 (corresponding to U.S. Pat. No. 5,805,253) and International Patent WO96/37804 (corresponding to European Patent Application No. 0783128A) disclose a liquid crystal display of bend alignment mode equipped with the optical compensatory sheet comprising discotic molecules. The compensatory sheet comprising discotic molecules remarkably improves the viewing angle of bend alignment mode display.
However, according to Japanese Patent Provisional Publication No. 11(1999)-316378, light at a particular wavelength often leaks to color the displayed image undesirably if the optical compensatory sheet comprising discotic molecules is used in the liquid crystal display of bend alignment mode. The publication reports that this undesired coloring is due to wavelength dependence of transmittance of the elliptically polarizing plate (layered composition of the polarizing device and the optical compensatory sheet).
In the publication, to avoid the undesired coloring, the polarizing membrane and the optical compensatory sheet are placed so that the average direction obtained by projecting normal of the disc plane of the discotic molecule onto the support may be essentially at the angle of 45° to the polarizing axis in the plane of polarizing membrane. The thus-placed optical compensatory sheet fully compensates the liquid crystal cell of bend alignment.